This is a good start for your detective work:
http://www.nissanforums.com/audio/58865-wiring-diagram-thread-useful-info.html
It's not the same car but same make. Also, looks like there are the same colored wires in the diagram. PPL/GRN+YEL=LEFT FRONT etc.
You need to have a voltmeter handy to test the for the +12V wire(s) [if you don't have one a bulb or something like that will do as long as you know which is ground (-)]. There are 2 of them but one is switched on ACC. There might be one for DIMMER as well, so make that 3 wires...depends. The diagram says PINK and BLUE. Attach (-) probe to ground then test for voltage on the wires. Once you have determined the POWER and GROUND...speakers are next.
Now you test for speakers. Be sure to figure out which is -/+ and keep them all in phase. Phase refers to the polarity. Test for polarity by taking a 1.5V batter and attaching to the speaker wires. If the cone pops out then the note the which wire is on the (+) of the batter as that's your (+) for the speaker. This ONLY works on speakers with woofers AND where you can verify direction of cone movement (not tweeters or mids). Another safer way is to attach to an external speaker with a known polarity. Attach the speaker of unknown polarity, listen, reverse polarity, and listen again. You do not want the "hollow" sound (180 degrees out of phase/reversed polarity).
It's not a big deal really. If your original stereo has 4 speakers then that eliminates 8 wires from the mess. Look on the old raido or the back of it. Often there's a diagram or legend. If all else fails take a trip to an auto parts store, find the correct manual, copy down the info and you're set.
I'm assuming you have all the wiring for the new stereo diagrammed out. Just TEST connect all the speakers in phase, then the power (don't mess that for a second else you;re gonna fry something!), turn down stereo, power on the stereo and use BAL/FADE to check speakers. Once you are sure, make permanent connections.
Personally, I'd shrink tube/solder all connections but crimp connections are fine. There's the HAT kind and the TUBE kind. There's also the splice types which cut into the wire but crimping is preferred unless you don't have a crimper. Probably don't need one as you all you need to do is mash the connections well.
Uhh...you assume all risks.
Sorry if I assumed you were not asking for the entire procedure, just the wiring diagrams.
Good luck!
EDIT: The Pioneer model # was not posted so can't help. Go the the pioneer site first. then Google the model number "xxx89xx wire diagram" or something like that. I doubt you will be left to guess for yourself on a well-known brand.
